Dust in the form of fibres and other particles is released from the web when creping off a soft crepe paper web, e.g. a tissue web, from a Yankee cylinder. To remove this dust it has been proposed in U.S. Pat. No. 4,019,953 (SE-381 899) to arrange a collection container below the area where the dust is produced. A compressed air pipe and a suction pipe are connected to this in order to remove dusty air drawn across the direction of transport of the web by jets of compressed air. Compressed air must be blown on since merely drawing off the dust by means of ventilation has little effect in view of poor distance action. Such a device is cumbersome, bulky and relatively inefficient since it only takes care of some of the dust falling down below the dust-producing area. The high speeds of modern tissue machines, namely in the region of near 25 m/s, contribute to the unsatisfactory result since dusty air is entrained along on both sides of the fast-moving tissue web which is delicate due to its low grammage.
The release of dust from the surface of a paper web is a problem when cutting and rewinding the paper web, for instance, U.S. Pat. No. 3,775,806 (SE-319 969) proposes an apparatus for dust-suction of the surface of a paper web. To avoid the web being drawn in towards the suction device and coming into contact with this, thus causing damage to the web, air is blown on at the same time as the suction. The proposed dust-suction device therefore comprises a horizontal sheet-metal channel, open at the top, which surrounds a rectangular blowing pipe and extends transversely across the web. The side of the blowing pipe facing the web has openings facing away from each other in order to direct air jets substantially parallel to the web both with and against the direction of transport of the web, thus fixing the distance between the web and dust suction device. Intermediate openings of the same type may be arranged to direct air jets towards the surface of the web to achieve a better dust suction effect by blowing dust off the surface. The air supplied and the dust entrained are withdrawn perpendicularly to the web, through the two gaps formed upstream and downstream of the blowing pipe, between it and the surrounding sheet-metal channel, to which a means for drawing off air is connected. The wall parts of the sheet-metal channel located nearest the web may be vertically movable up and down enabling them to be set in such a manner that the dust-carrying air blown on does not flow past the suction gaps, neither does the web scrape against said wall parts and become damaged. The dust-suction effect achieved with the arrangement according to U.S. Pat. No. 3,775,806 does not, however, fulfil modern requirements with regard to also taking care of the dust that accompanies the air around a fast-moving paper web. Furthermore, the arrangement is both bulky and expensive to purchase and to operate due to the air flows required by the blow-on technique.
U.S. Pat. No. 4,906,333 (SE-B 459 105) proposes an apparatus for removing dust in the boundary layer of a creped web. This apparatus comprises a web-wide hood and a flat cover plate that closes the hood while defining a space in which subatmospheric pressure prevails, and forming a suction gap between the cover plate and an angled strip at the front side edge of the hood. The cover plate is located immediately next to the creped web so that, during transport, it is held close to the cover plate and said suction gap will be located in the dust-containing boundary layer. A lower part of the angled strip faces towards the space and lies parallel with the cover plate in order to define the suction gap, thereby ensuring that its width remains constant in the direction of flow of the air. A suction gap formed in this way easily becomes clogged with dust after a relatively short time in operation and must therefore be cleaned at regular intervals. Another problem is that the web may be damaged by the front edge and flat cover plate, causing a break in the web. The problem is aggravated since the web is drawn by the air flow up towards the front edge of the cover plate. Said space in the hood is irregular in shape, with several corners and edges that detrimentally affect that air flow so that local whirls of air containing dust occur, the flow of the air towards the outlet gradually deteriorates and dust collects along the walls near said corners and edges, which in turn results in poorer suction effect so that the suction gap becomes more easily clogged by dust particles for this reason as well.
However, the problem of suction inlets and channels becoming clogged is general to all known apparatus for removing dust released during treatment of a moving web of fibre material. Installing nozzles inside the actual suction channel has been suggested so that air can be blown in the opposite direction through the gap at regular intervals, in order to remove collections of dust when the suction source has been disconnected. Such interruptions in operation are undesirable since dust is produced continuously and will be blown out into the surroundings, causing further deterioration of the working environment.
U.S. Pat. No. 5,466,298 discloses a dust suction system with upstream and downstream curved surfaces in a suction inlet opening which is directed vertically downwardly. Pressurized air is directed to the web and flows along the curved downstream wall of the suction inlet opening by the Coanda effect.
Accordingly, one object of the invention is to provide an improved apparatus for taking care of and removing the dust released during treatment of a moving web of fibre material, thereby achieving an improved working environment for the operating personnel.